Lithographic apparatus and device manufacturing method

ABSTRACT

The present invention relates to the method of reducing contamination of an immersion liquid in a lithographic apparatus when a closing surface is used to confine liquid in a liquid supply system. To avoid or reduce particulate contamination caused by the closing surface colliding with the liquid supply system, the closing surface is maintained at a distance away from the liquid supply system such that there is no collision between the closing surface and the liquid supply system, but the liquid is nonetheless confined.

FIELD

The present invention relates to a lithographic apparatus and a devicemanufacturing method, in particular an immersion lithographic apparatuswhich contains a liquid between a projection system and a substrate tobe exposed and a related immersion device manufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application WO 99/49504, hereby incorporatedin its entirety by reference. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet OUT after having passedunder the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in- and out-lets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

SUMMARY

Between exposures of substrates, a substrate finished being exposed isswapped with a substrate to be exposed. In order to do this in animmersion lithographic apparatus, a substrate positioned below theliquid supply system and the immersion liquid may be replaced by aclosing surface. The closing surface enables, for example, a substrateto be swapped with another substrate without having to remove the liquidadjacent the substrate. The closing surface may allow the liquid flowwithin the liquid supply system to be maintained uninterrupted and tokeep the surface of a final optical element of the projection systemcontinuously wet. While the closing surface may be used for swapping onesubstrate for another, it may also be used in other applications such asmaintenance on the substrate table, measurements of the substratewithout liquid, etc. where it is desired to separate the liquid from thesubstrate and/or substrate table.

In an embodiment, the closing surface may be coupled with the liquidsupply system using a vacuum on the bottom surface of a liquidconfinement structure of the liquid supply system. A potential problemwith this and other methods which may be used to couple the closingsurface, such as mechanical methods, is that when the closing surfaceimpacts the liquid supply system, particles may be dislodged from eitherthe closing surface or the liquid supply system and enter the liquid.These particles may contaminate the liquid supply system and the liquid,and may contaminate the final optical element of the projection systemand/or the substrate and/or they may partially block the light to causeprinting defects. Contamination on the final optical element, on thesubstrate and/or in the liquid, where it may get in the path of theexposing projection beam, may reduce the accuracy of the exposure of thesubstrate. Furthermore, deposition of contaminants on the substrate maycause problems in other processing steps following the exposure process,such as etching, deposition, etc.

Accordingly, it would be advantageous, for example, to have a closingsurface positioning system which may reduce the risk of contamination ofthe immersion liquid and any or all of the surfaces adjacent orcontacting the liquid.

According to an aspect of the present invention, there is provided alithographic apparatus, comprising:

a projection system configured to project a patterned radiation beamonto a substrate supported by a substrate table;

a liquid supply system configured to supply a space between theprojection system and the substrate with a liquid;

a closing surface configured to provide a confining surface for liquidsupplied by the liquid supply system in place of the substrate; and

a closing surface positioning device configured to create and maintain agap between the liquid supply system and the closing surface when theclosing surface is used to confine the liquid supplied by the liquidsupply system.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising:

supplying a liquid to a space between a projection system of alithographic projection apparatus and a substrate using a liquid supplysystem;

creating and maintaining a gap between the liquid supply system and aclosing surface when the closing surface confines the liquid supplied bythe liquid supply system in place of a substrate; and

projecting a patterned beam of radiation through the liquid onto thesubstrate when the substrate is used as a confining surface for theliquid supplied by the liquid supply system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 6 depicts a liquid confinement structure of a liquid supply systemwith a closing plate;

FIG. 7 depicts the liquid confinement structure of the liquid supplysystem depicted in FIG. 2 with the closing plate positioned according toan embodiment of the present invention;

FIG. 8 depicts the liquid confinement structure and closing plateorientation of FIG. 7 according to an embodiment of the presentinvention; and

FIG. 9 depicts the liquid confinement structure and closing plateorientation of FIG. 7 according to another embodiment of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) EL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using patterning device alignment marks M1, M2 and substratealignment marks P1, P2. Although the substrate alignment marks asillustrated occupy dedicated target portions, they may be located inspaces between target portions (these are known as scribe-lane alignmentmarks). Similarly, in situations in which more than one die is providedon the patterning device MA, the patterning device alignment marks maybe located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. Such a system is shown inFIG. 5. The liquid confinement structure is substantially stationaryrelative to the projection system in the XY plane though there may besome relative movement in the Z direction (in the direction of theoptical axis). A seal is formed between the liquid confinement structureand the surface of the substrate. In an embodiment, the seal is acontactless seal such as a gas seal. Such a system with a gas seal isdisclosed in U.S. patent application Ser. No. 10/705,783, herebyincorporated in its entirety by reference.

FIG. 5 depicts an arrangement of a reservoir 10, which forms acontactless seal to the substrate around the image field of theprojection system so that liquid is confined to fill a space between thesubstrate surface and the final element of the projection system. Aliquid confinement structure 12 positioned below and surrounding thefinal element of the projection system PL forms the reservoir. Liquid isbrought into the space below the projection system and within the liquidconfinement structure 12. The liquid confinement structure 12 extends alittle above the final element of the projection system and the liquidlevel rises above the final element so that a buffer of liquid isprovided. The liquid confinement structure 12 has an inner peripherythat at the upper end preferably closely conforms to the shape of theprojection system or the final element thereof and may, e.g., be round.At the bottom, the inner periphery closely conforms to the shape of theimage field, e.g., rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the liquid confinement structure 12 and the surface of thesubstrate W. The gas seal is formed by gas, e.g. air, synthetic air, N₂or an inert gas, provided under pressure via inlet 15 to the gap betweenliquid confinement structure 12 and substrate and extracted via outlet14. The overpressure on the gas inlet 15, vacuum level on the outlet 14and geometry of the gap are arranged so that there is a high-velocitygas flow inwards that confines the liquid. It will be understood by theperson skilled in the art that other types of seal could be used tocontain the liquid such as simply an outlet to remove liquid and/or gas.

The gas seal may also be used to create a gap between the liquid supplysystem and either the closing surface or the substrate table.

FIG. 5 shows a liquid confinement structure 12 of a liquid supply systemwith a substrate W positioned below the immersion liquid 11. When, forexample, this substrate is to be swapped with another substrate to beexposed next, the substrate W is replaced by a closing plate 20 as shownin FIG. 6. The closing plate 20 need not have a plate-like shape butonly needs to provide a surface that can confine liquid. Thus, closingplate 20 can also be referred to as a closing structure or a closingsurface.

FIG. 6 shows an orientation of the liquid confinement structure 12 andclosing plate 20. A low pressure provided through outlet 14 as shown inFIGS. 5 and 6 is used to urge the closing plate 20 against the liquidconfinement structure 12. When the closing plate 20 is urged against theliquid confinement structure 12 by outlet 14, particulate contaminationof the immersion liquid 11 may occur because of particles being releasedfrom the surface of the closing plate 20 and/or the liquid confinementstructure 12, predominantly initiated by their physical contact.Although there is an amount of liquid flow 13, it does not circulate theliquid 11 close to the surface of the closing plate 20 and particles areunlikely to be washed away as long as the closing plate is in position.

FIG. 7 shows the closing plate 20 being maintained at a distance fromthe liquid confinement structure 12. In this way, there are at least twoadvantages—the first is that particles are unlikely to be released fromthe surfaces of the closing plate 20 and/or the liquid confinementstructure 12 and the second is that any particles which are released andpresent in the immersion liquid 11 may be removed by the circulation ofthe liquid, particularly via outlet 14.

There are closing plate positioning devices by which the closing platemay be held a distance away from the liquid supply system (e.g., theliquid confinement structure). A first such device is the outlet 14 andthe inlet 15 of the liquid confinement structure which may be used tomaintain an equilibrium state between the low pressure of outlet 14 andthe gas flow provided through inlet 15 as shown in FIG. 5. Thisequilibrium may be used not only to contain the immersion liquid 11, butalso to cause the closing plate to “float” just below the liquidconfinement structure.

FIG. 8 shows the closing plate 20 being maintained a distance away fromthe liquid confinement structure 12 by the use of a gripper 22. Thegripper may be any mechanical device that maintains the position of theclosing plate 12 for the desired duration (e.g., of the substrate swap).In FIG. 8, the gripper 22 is connected at one end to the closing plate20 and at the other end to the liquid confinement structure 12. Theother end of the gripper 22 may connected to other structures than theliquid confinement structure 12, such as the substrate table WT or aframe of the lithographic apparatus. An alternative may be a pin systemcomprising one or more pins that push the closing plate 20 away from itsresting position on a substrate table WT for example, or any othermechanical device.

FIG. 9 shows an alternative embodiment of the present invention. Inparticular, the closing plate 20 is maintained a distance away from theliquid confinement structure 12 by use of magnets 24. The magnets may beused in conjunction with the low pressure/gas flow equilibrium describedabove, or they may be balanced against one or more other magnets. Forexample, magnets with opposing poles may be balanced against magnetswith the same poles facing each other to cause the closing plate 20 to“float”. In an embodiment, a magnet may be provided on each of theliquid supply system and the substrate table to facilitate maintainingthe closing plate 20 a distance away from the liquid confinementstructure 12. It will be appreciated that other similar methods ofholding the closing plate 20 a distance away from the liquid supplysystem, such as electrostatics, etc.

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the present invention may be applied to anyimmersion lithography apparatus, such as those types mentioned above,and whether the immersion liquid is provided in the form of a bath oronly on a localized surface area of the substrate. A liquid supplysystem is any mechanism that provides a liquid to a space between theprojection system and the substrate and/or substrate table. It maycomprise any combination of one or more structures, one or more liquidinlets, one or more gas inlets, one or more gas outlets, and/or one ormore liquid outlets, the combination providing and confining the liquidto the space. In an embodiment, a surface of the space may be limited toa portion of the substrate and/or substrate table, a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus, comprising: a projection system configuredto project a patterned radiation beam onto a substrate supported by asubstrate table; a liquid supply system configured to supply a spacebetween the projection system and the substrate with a liquid; a closingsurface configured to provide a confining surface for liquid supplied bythe liquid supply system in place of the substrate; and a closingsurface positioning device configured to create and maintain a gapbetween the liquid supply system and the closing surface when theclosing surface is used to confine the liquid supplied by the liquidsupply system, wherein, in use at a point in time, (i) no portion of theclosing surface comes in contact with the liquid supply system, (ii) theclosing surface is held only by the liquid supply system, and (iii)liquid flows within the gap.
 2. The apparatus according to claim 1,wherein the closing surface positioning device comprises an inlet andoutlet configured to create an equilibrium state between forces exertedby a fluid supplied by the inlet and a low pressure provided by theoutlet.
 3. The apparatus according to claim 1, wherein the closingsurface is formed on a closing plate, and wherein the closing surfacepositioning device comprises a gripper configured to grip the closingplate on a face facing other than towards the liquid in the liquidsupply system.
 4. The apparatus according to claim 3, wherein gripper isattached to the liquid supply system.
 5. The apparatus according toclaim 1, wherein the closing surface positioning device comprises a pinsystem configured to displace the closing surface from the substratetable.
 6. The apparatus according to claim 1, wherein the closingsurface positioning device comprises a magnet on each of the liquidsupply system and the closing surface.
 7. The apparatus according toclaim 6, wherein a magnet on each of the liquid supply system and theclosing surface having opposing poles is balanced against a magnet oneach of the liquid supply system and the closing surface having the samepoles.
 8. The apparatus according to claim 1, wherein the closingsurface positioning device comprises a magnet on each of the liquidsupply system and the substrate table.
 9. The apparatus according toclaim 1, wherein the liquid that flows within the gap circulates. 10.The apparatus according to claim 1, wherein the liquid that flows withinthe gap flows to an outlet.
 11. A device manufacturing method,comprising: supplying a liquid to a space between a projection system ofa lithographic projection apparatus and a substrate supported by asubstrate table using a liquid supply system; creating and maintaining agap between the liquid supply system and a closing surface when theclosing surface confines the liquid supplied by the liquid supply systemin place of a substrate; and projecting a patterned beam of radiationthrough the liquid onto the substrate when the substrate is used as aconfining surface for the liquid supplied by the liquid supply system,wherein, at a point in time, (i) no portion of the closing surface comesin contact with the liquid supply system, (ii) the closing surface isheld only by the liquid supply system, and (iii) liquid flows within thegap.
 12. The method according to claim 11, wherein creating andmaintaining the gap comprises creating an equilibrium state betweenforces exerted by a fluid flow and a low pressure.
 13. The methodaccording to claim 11, wherein the closing surface is formed on aclosing plate, and wherein creating and maintaining the gap comprisesgripping the closing plate on a face facing other than towards theliquid in the liquid supply system.
 14. The method according to claim13, wherein gripper is attached to the liquid supply system.
 15. methodaccording to claim 11, wherein creating and maintaining the gapcomprises displacing the closing surface from the substrate table usinga pin.
 16. method according to claim 11, wherein creating andmaintaining the gap comprises magnetically maintaining the gap using amagnet provided on each of the liquid supply system and the closingsurface.
 17. method according to claim 16, wherein maintaining the gapusing a magnet provided on each of the liquid supply system and theclosing surface comprising maintaining the gap using a magnet on each ofthe liquid supply system and the closing surface having opposing polesbalanced against a magnet on each of the liquid supply system and theclosing surface having the same poles.
 18. method according to claim 11,wherein creating and maintaining the gap comprises magneticallymaintaining the gap using a magnet provided on each of the liquid supplysystem and the substrate table.
 19. The method according to claim 11,wherein the liquid that flows within the gap circulates.
 20. The methodaccording to claim 11, wherein the liquid that flows within the gapflows to an outlet.